L-Hydroxyproline methyl ester hydrochloride, CAS 40216-83-9, molecular weight 181.62 g/mol, is supplied as a white to off-white crystalline solid for use as a chiral C5 pyrrolidine scaffold. The following application scenarios are confined to existing downstream routes where the trans-4-hydroxy-L-proline configuration is retained and the methyl ester and hydrochloride functionalities participate in defined chemical transformations.

Chiral Gateway Intermediates for Pyrrolidine-Based API Synthesis

In pyrrolidine-based API intermediate routes, the hydrochloride is converted to N-acetyl-4-hydroxy-L-proline and related 4-substituted proline scaffolds through sequential neutralisation, N-acylation, ester hydrolysis, and recrystallisation. Compliance boundary: when the ester is used as a registered GMP starting material, batch release follows ICH Q7 sections 7.1 and 11.1; residual solvents are tested according to Ph. Eur. 2.4.24, water by Ph. Eur. 2.5.12, and optical rotation by Ph. Eur. 2.2.7. Charge ratio: the hydrochloride is neutralised in situ with 2.0-2.2 mol sodium bicarbonate per 1.0 mol substrate; acetic anhydride is introduced at 1.05-1.15 mol per mole of free amine to compensate for aqueous hydrolysis, and sodium hydroxide for subsequent ester saponification is limited to 1.0-1.1 mol to avoid pyrrolidine ring opening. Production route: the neutralised methyl ester is acetylated at 0-5°C in a glass-lined reactor with retreat-curve impeller at 120-150 rpm; after methyl ester hydrolysis at 20-25°C, the reaction mass is acidified to the isoelectric point and crystallised from aqueous ethanol. Batch failure on scale has been observed when the pH during acetylation drifts above 8.5, generating N,O-bis-acetyl by-products that co-crystallise and depress optical rotation below release specification. Finished product types: oxaceprol API and N-acetyl-4-hydroxy-L-proline intermediates, as well as 4-O-alkylated pyrrolidine building blocks for further chiral API synthesis.

How Is the Hydrochloride Converted into Fmoc-Hyp(tBu)-OH for Solid-Phase Peptide Synthesis?

A batch-scale protection sequence begins with dissolution of the hydrochloride in water/dioxane followed by portion-wise addition of sodium carbonate to pH 8.0-8.5 at 0-5°C. Fmoc-OSu is charged at 1.05-1.10 mol per 1.0 mol amine; the pH is kept below 8.8 because premature saponification of the methyl ester generates Fmoc-Hyp-OH, a contaminant that co-elutes with the desired Fmoc-Hyp-OMe on normal-phase TLC and is difficult to remove by single crystallisation. After extraction into methyl tert-butyl ether and solvent switch to dichloromethane, tert-butyl etherification is performed with isobutylene at 2-4 mol per hydroxyl equivalent in a sealed glass pressure vessel at 0.2-0.4 MPa and 10-15°C using acid catalysis; the pressure profile is monitored because loss of isobutylene through the rupture disk indicates overpressure and incomplete conversion. The methyl ester is then saponified with lithium hydroxide at 0.0-1.0°C; the stoichiometry is held at 1.0-1.05 mol LiOH per 1.0 mol ester to preserve the Fmoc group. Compliance boundary: GMP production for peptide building blocks is governed by ICH Q7; residual solvents follow USP <467> and Ph. Eur. 2.4.24; water content is determined by Ph. Eur. 2.5.12; chiral purity is confirmed by HPLC on a chiral stationary phase. Production route detail: isolation involves acidification to pH 2.5-3.0, extraction into ethyl acetate, brine washing, drying over magnesium sulfate, and crystallisation from ethyl acetate/n-heptane at -10 to -5°C. Vacuum double-cone drying at 35-40°C and ≤ -0.08 MPa reduces residual ethyl acetate below 500 ppm. Finished product types: Fmoc-trans-4-hydroxy-L-proline tert-butyl ether for solid-phase peptide synthesis, collagen-mimetic peptide APIs, and diagnostic peptide standards.

Reported organocatalytic asymmetric aldol additions using hydroxyproline-derived catalysts frequently begin with the hydrochloride as the chiral pool precursor for O-silylation and amine modulation. Compliance boundary: for non-GMP R&D and fine chemical supply, release is typically under ISO 9001:2015; residual metals are controlled by USP <233>; identity and diastereomeric purity are confirmed by ¹H NMR at 400 MHz and chiral HPLC. Formula addition ratio: the final O-silylated proline derivative is used at 5-20 mol% relative to the aldehyde electrophile in direct aldol reactions; catalyst loadings above 20 mol% are avoided because the unprotected hydroxyl of hydroxyproline methyl ester competes for enamine formation and broadens the aldol adduct distribution. Downstream production: the hydrochloride is neutralised to the free amine, then O-protected with tert-butyldimethylsilyl chloride in N,N-dimethylformamide containing imidazole at 20-25°C; after aqueous quench and extraction into ethyl acetate, the crude silyl ether is distilled or chromatographed. Industrial bottlenecks include residual DMF retention above 0.1 wt% after rotary evaporation, which interferes with catalyst crystallisation. Finished product types: chiral organocatalysts for asymmetric aldol, Michael addition, and Mannich routes in pharmaceutical R&D and fine chemical synthesis.

When Collagen-Mimetic Peptide Hydrogels Require Defined Hydroxyproline Content

Triple-helix formation in collagen-mimetic peptides is governed by the Pro-Hyp-Gly repeating sequence; the hydrochloride enters the route as the precursor to Fmoc-Hyp(tBu)-OH or as a solution-phase amino acid ester after neutralisation. Compliance boundary: biomaterial use requires ISO 10993-5 cytotoxicity testing for extractables, ISO 13485:2016 for medical device quality management, and for injectable hydrogels USP <71> sterility testing; residual solvents in the finished peptide are controlled by USP <467>. Formula addition ratio: hydroxyproline is incorporated at 33 mol% in the triple-helix repeat unit, with Pro-Hyp-Gly charged at 1:1:1 molar ratio during solid-phase assembly; final hydrogel formulations typically contain peptide at 0.5-3.0 wt% in pH 7.4 phosphate-buffered saline. Downstream production: peptides are assembled on a 2-chlorotrityl chloride resin at 0.10-0.25 mmol/g loading, cleaved with trifluoroacetic acid/triisopropylsilane/water, precipitated in cold diethyl ether, purified by preparative C18 HPLC with 0.1% trifluoroacetic acid mobile phase, and lyophilised. Batch-to-batch variance in triple-helix melting temperature has been traced to incomplete removal of TFA counterions after HPLC; residual trifluoroacetate above 0.5 wt% disrupts hydrogen bonding and depresses the circular dichroism melting transition. Self-assembly is carried out by dissolving lyophilised peptide in cold 20 mM acetic acid at 4°C, then dialysing against phosphate buffer; triple-helix formation is confirmed by circular dichroism with a 0.1 mm path length cell and a scan rate of 10 nm/min from 190 nm to 260 nm. Finished product types: injectable collagen-mimetic hydrogels, extracellular matrix coatings for cell culture, wound healing scaffolds, and lyophilised peptide raw materials for tissue engineering.

For topical acyl hydroxyproline derivative synthesis, the methyl ester hydrochloride is consumed as an intermediate in Schotten-Baumann N-acylation with fatty acid chlorides; the methyl ester is subsequently hydrolysed or retained depending on the desired emollient polarity. Compliance boundary: cosmetic ingredient manufacturing in the EU falls under Regulation (EC) No 1223/2009 and ISO 22716:2007; residual fatty acid chloride is controlled by HPLC and limited to specified trace levels under safety assessment; the final product must not contain prohibited Annex II substances. Formula addition ratio: the derived hydroxyproline acyl derivative is formulated in leave-on emulsions at 0.1-2.0 wt%; the synthesis itself uses fatty acid chloride at 1.0-1.2 mol per 1.0 mol neutralised methyl ester, with aqueous alkali maintained at pH 7.5-8.0. Downstream production: the hydrochloride is neutralised in water; the fatty acid chloride is added dropwise at 0-5°C to a stirred reactor, then the mixture is acidified and extracted into ethyl acetate; after solvent evaporation, the product is crystallised or chromatographed. Production-scale failure occurs when the exotherm during fatty acid chloride addition exceeds 15°C because residual methyl ester hydrolysis increases and the acyl derivative forms an intractable emulsion. Finished product types: hydroxyproline-containing acyl amino acid derivatives and emollients for leave-on facial serums, scalp treatment lotions, and rinse-off hair conditioning formulations.